mount: dump one file system only once (v2)

A file system can be bind-mounted a few times and some of these mounts
can be non-root. We need to find one of root mounts and dump it.

v2: don't forget to check pm->dumped and pm->parent
    don't dump a root file system, it's always external for now.

Reported-by: Saied Kazemi <saied@google.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>